A form measuring instrument with a tip of a stylus that is brought into contact with a workpiece surface and moved therealong in a predetermined measurement direction has been used to measure the workpiece surface (see Patent Literature 1: JP-A-6-129810).
In such a contact-type form measuring instrument, a contact condition of the tip of the stylus on a workpiece surface comes to affect a measurement result as the measurement accuracy improves.
For instance, during the scanning movement of the tip of the stylus on a workpiece surface, the tip of the stylus sometimes temporarily sticks to the workpiece surface due to friction therebetween and unsticks in the next moment, resulting in a non-smooth movement (i.e., stick-slip).
Further, when a workpiece surface has a step, a projection or the like, the tip of the stylus having been moved along the workpiece surface is likely to be caught by such a portion to be hindered from continuously moving along the workpiece surface thereafter.
In order to avoid the above-described influence of friction, some recent form measuring instruments are designed to exhibit a reduced measurement force for pressing the tip of the stylus on a workpiece surface.
It is also disclosed that a stylus has a tip subjected to a low-friction coating using a diamond-based material or the like to suppress friction coefficient (see Patent Literature 2: JP-A-2003-240700).
Further, it is disclosed that a tip of a stylus is vibrated during a movement along a workpiece surface, thereby reducing a friction force between the tip of the stylus and the workpiece surface (see Patent Literature 3: JP-A-2001-91206).
However, a reduction in a measurement force to reduce a friction force between a tip of a stylus and a workpiece surface as described above is accompanied by a decrease in the rigidity of a contact portion, so that a high-speed measurement becomes difficult to perform and thus operating efficiency is lowered.
Additionally, a low-friction coating using a diamond-based material or the like generally costs a lot and thus increases the cost of a form measuring instrument.
When a method such as vibrating a tip of a stylus is employed, a form measuring instrument is inevitably subject to structural limitations.
Specifically, when a form measuring instrument has such an arrangement that a stylus having one end provided with a tip is cantilevered, a friction force between the tip of the stylus and a workpiece surface can be reduced by vibrating a tip-side of the stylus. According to the above arrangement, a profile of the workpiece surface with which the tip of the stylus is in contact is detected by detecting a displacement of a portion of the stylus opposite to the tip-side, so that even when vibration is applied to the tip-side of the stylus to cause a bending deformation of the tip-side of the stylus along with the vibration, the measurement result is unaffected because the measurement is performed in the vicinity of the tip of the stylus.
In contrast, in a form measuring instrument having such an arrangement that a stylus having one end provided with a tip is supported by a rotary measurement arm or stylus holder and the rotation of the stylus holder is detected to measure a displacement of the tip of the stylus, for instance, when only the stylus is vibrated to undergo a bending deformation, a relationship between a displacement of the tip of the stylus and a rotation angle of the stylus holder cannot be maintained and thus the displacement of the tip of the stylus cannot be detected by a detector near the stylus holder.
A lot of form measuring instruments employ such an arrangement that a tip of a stylus is displaced by rotation for a great flexibility in the installation position of a detector. With such an arrangement, detection may be performed, for instance, at the backside of the tip of the stylus instead of the distal side of the stylus. In view of the above, for a form measuring instrument with a tip of a stylus that is displaced by rotation, it has been strongly demanded to stabilize the contact condition between a workpiece surface and the tip of the stylus.